Clock generation circuits are employed in many applications, including those involving specialized cryptographic processors. For example, FIG. 1 is a block diagram of a conventional random number generator circuit 100 which includes a clock generation circuit 110. In the random number generator circuit 100, one or more ring oscillators are employed in the clock generation circuit 110 to produce a non-stable output. The output of the clock generation circuit 110 is then combined with the feedback of a linear feedback shift register (LFSR) circuit 120. The non-stable output of the clock generation circuit 110 introduces unknowns into the LFSR 120 output such that the LFSR 120 output has random properties. In particular, the LFSR 120 output may be truly random or may be merely "pseudo"-random. (As used in this specification, including in the claims, the term "random" is meant to encompass both.) The random output of the random number generator circuit 100 is provided to a cryptographic processor 130, via a random number generator register 140. The processor 130 uses the random output to create a unique session key for encrypting data.
Conventionally, to determine if the ring oscillators of the clock generation circuit 110 are operating properly, the ring oscillator output signals are provided to the outside of the random number generator circuit 100 package. However, security is critical in cryptographic applications and an intruder could more accurately predict the output of the random number generator circuit 100 with the information gained from examining the ring oscillator output signals.